已知①2C(s)+O2(g)====2CO(g);ΔH=-221.0 kJ·mol-1②2H2(g)+O2(g)====2H2O(g);ΔH=-483.6 kJ·mol-1。则制备水煤气的反应C(s)+ H2O (g)====CO(g)+H2(g);ΔH为